1. Field of the Invention
The present invention relates to a calender for webs of paper having at least two rollers to form a roll gap therebetween. More specifically, the present invention relates to a calender having a "hard" roller which is made of metal and has a smooth surface, and a "soft" roller which has a coating made of flexible plastic.
2. Discussion of the Related Art
Calenders having a hard and soft roller are commercially available and are known as "soft calenders". The smoothness and gloss of the webs of paper is created by the smooth surface of the hard roller. A uniform compression of the paper is created because of the flexible coating on the soft roller. To provide both sides of the paper with gloss and smoothness, the web of paper must run through at least two roll gaps, where the hard rollers of the two gaps are placed on opposites sides of the web of paper. Soft calenders can also be used as in-line calenders in a paper machine because the plastic coating of the soft roller is substantially less sensitive than a coating made of paper. In other words, because the soft roller has to be taken out of service and reworked only in exceptional cases, soft calenders can also be used in paper machines.
German reference DE 41 26 232 A1 teaches that a calender roller that has a plastic coating can be provided on the outside with a thin metallic layer of material having good thermal conductivity properties to direct heat to or away from the roll gap. In particular, this layer can be made of a metal that has a glassy smooth or highly glossy surface.
In addition, German reference DE-U-84 06 019 teaches that a hollow, cylindrical roller body can be made from a carbon-fiber reinforced plastic. The plastic roller has a jacket, which is coated with a metal that is resistant to centrifugal force and has a thickness of 10 to 1,000 .mu.m.